Continuing use of stable isotope dilution liquid chromatography/mass spectrometry (LC/MS) have produced a number of significant findings: 1) Measurement of cortisol levels and cortisol production rates a) the value of urinary free cortisol in anorexia nervosa (n=5) averages 97+/-46 (mu)g/day vs normal values of 42+/-17 (mu)g/day. Morning serum cortisol levels in the anorexia nervosa patients averaged 32+/-12 (mu)g/dl (normal 10-20 (mu)g/dl). These data suggest that, despite high calcium intakes, significant abnormalities in calcium metabolism exist in adolescents with this eating disorder [See project ZOI HD 01400-08 LTPBI. 2) The earlier findings of daily FPR being significantly lower than accepted values has raised issues of appropriate therapies. Bioavailability studies using oral hydrocortisone and a tri-dutero tracer to determine the appearance of cortisol in plasma indicate absorption of 56+/-10% in 4 patients with adrenal insufficiency. These data may be used to determine appropriate oral replacement levels of cortisol. 3) Measurement of the endogenous production rate of 25-OH vitamin D3 (25OH-D) has been accomplished using 6, 19, 19(- 2)H3-25-OH-vitamin D3 as a tracer. Using a single pool model and bolus infusion of tracer at -5% of the plasma pool level of preliminary results yield a value for the half life of 25D of 11+/-1 days (n=5). Earlier studies using radio-labelled tracers have yielded values of the half life of 25OH-D of about 20 days in humans and have suggested that, in rats, the value varies with dietary fiber. We undertook the measurement of the half life of 25OH-D in humans along with the investigation of the effects of dietary fiber on calcium absorption and 25OH-D metabolism. Preliminary results show a) half life that is reduced by about 50% from earlier results, and b) no perturbation of the 25OH-D metabolism by high levels of dietary fiber.